New and Upcoming Star Catalogues
Obsolete star catalogues (AGK3, SAO) :
Under this heading are the two venerable equinox B1950.0 catalogues: the AGK3 and the SAO. The AGK3 has about 200,000 stars north of -2 Dec, containing positions, proper motions, and photo-blue magnitudes complete to about blue magnitude 8.5, but with many stars down to mag. 12 or so. This used to be the catalogue of choice for the northern hemisphere, since it had twice as many stars as the SAO catalogue, and the positions were more accuracte. At present, the positions in the AGK3 are good to about 0".75. The SAO catalogue was compiled in the 1960s from a large number of sources, reduced as well as possible (at that time) to a common system. The catalogue contains about 260,000 stars for the whole sky (less dense than the AGK3 in the north). It was intended for satellite tracking (from the ground) and also for attitude-control on orbiting spacecraft. Despite having these origins as an engineering tool, the compilers tried to make it useful for astronomy as well. At present the positions in the SAO have rather large errors, typically around 1".2 in the north, and progressively worse south of about -20 Dec, with many cases of errors up to 10" in the far-southern sky. The reason for this is the sheer age of the positions that went into the catalogue, which have a mean ending epoch around 1940. As a result, when the proper motions derived from the older data are extrapolated to the present, substantial errors creep in, even if the original positions are good.
Star catalogues that are about to be superceded (PPM, ACRS, GSC v1.1) :
For most purposes there are again two "catalogues of precision" in use for doing astrometry, making charts, etc. One is the ACRS ("Astrographic Catalog Reference Stars"), which was produced about 1990 by the U. S. Naval Observatory. It is an all-sky catalogue of about 380,000 stars, compiled as was the SAO from a large number of older sources. In this case, the analysis of the input data was much better than before, and more stars could be included as well from modern observations. The current accuracy of the positions is about 0".3, a factor of four better than the SAO; the motions are factors of six to ten more accurate.
The other catalogue is the PPM ("Positions and Proper Motions"), which was compiled in a similar way by the Astronomisches Rechen-Institut in Heidelberg. This catalogue contains altogether about 470,000 stars with mean errors similar to the ACRS. Since many people get confused: in the northern sky, the PPM lists the photo-blue magnitudes directly from the AGK3. These magnitudes will thus be about 0.5 to 1.0 (typically) fainter than visual magnitudes. In the south, the magnitudes are a mix of photo-visual and photo-blue magnitudes, and even some real visual estimates surviving from hundred-year-old catalogues such as the Cordoba Durchmusterung. There is a code in the PPM-South that tells the source of the magnitude for each star. In all cases, the magnitudes scatter from "truth" by around +/- 0.3-0.5 mag. For most of the naked-eye stars, however, no matter where in the sky they are, accurate photoelectric V magnitudes are given (to 0.1 mag. precision).
Far more comprehensive, but of lower accuracy, is the Space Telescope Guide Star Catalogue (GSC). The GSC version 1.1, which now appears in many software packages intended for the amateur market. It contains about 16,000,000 'things' from digitally-scanned Schmidt plates. Most of these are stars and galaxies, but also emulsion flaws, asteroids, parts of galaxies, pieces of diffraction spikes emanating from bright stars; there are also bits of dust, dandruff flakes, and whatnot included in the GSC. It is also an engineering product (to point and track Space Telescope), not a true astronomical catalogue, no matter that both professionals and amateurs are using as if it were the latter. For the star positions, the GSC v1.1 used as the reference net the AGK3 in the north and the SAO in the south, there being no other reference catalogues available when the plates were scanned in the 1980s. On top of this problem, the equations used to convert the x,y pixel coordinates of the scans into RA and Dec did not adequately model the distortions of the Schmidt plates. As a result, in the corners of the plates, the star positions are frequently off by 2"-3", plus having the general errors caused by the old star catalogues. Mean errors, however, are about 0".8 at present.
It is worth re-emphasizing that the GSC is _not_ an inventory of the sky, only a list of stars for controlling Space Telescope. Thus there are plenty of fairly bright stars missing from the catalogue that will mislead you if you compare charts made from them with views at the eyepiece. (It has already happened that someone reported as a nova a mag. 8 BD star not contained in the GSC.) Along the Milky Way, the catalogue starts to become incomplete around mag. 11.5, even though there are a lot of mag. 13 and 14 stars given in the same area. And there are certainly plenty of mag. 10 stars missing from the GSC in the Milky Way.
This is a good place to discuss the magnitudes in the GSC: they're useless! In the north (specifically north of about +3 Dec), the GSC derives from short-exposure yellow-light plates taken in a special survey at Palomar. South of +3 Dec, the deep blue-light survey plates from the UK Schmidt in Australia were used. Along the southern Milky Way, in order to avoid severe crowding problems, a series of V-band plates was taken with the UK Schmidt. Special plates were taken for such areas as the Magellanic Clouds, M31, etc. Quite a mix of plate material! The magnitudes were calibrated with sequences of about six stars near the centers of each field. In the immediate region of the those sequences, say within a circle 1 degree across, the GSC magnitudes are often pretty good. Outside that area, all bets are off, and the GSC magnitudes scatter over a range of two to three whole magnitudes. Sometimes they're okay, but usually they're off by half a magnitude or more. And since they're based on only a single plate, the internal errors are no better than +/- 0.3-0.4 mag. at best. In the north, of course, the magnitudes are in the yellow, but in the south they're blue-light magnitudes (i.e. fainter than visual for most stars). However, except for the southern near-Milky Way regions, _none_ of the magnitudes are on the standard V ("visual") system. In the north, they're redward of visual, so red stars come out too bright; in the south, they're blue, so red stars come out too faint. The bottom line is not to use the GSC magnitudes for anything. No, not anything.
Current star catalogues (circa September 1997; TAC, GSC v1.2, A1.0) :
The ACRS mentioned above was compiled mainly to rereduce measurements from the "Astrographic Catalogue" (AC). The AC and accompanying "Carte du Ciel" (Sky Atlas) was a mammoth project from the turn of the Century to compile a catalogue of stars down to about mag. 11.5, and a photographic star atlas down to about 14th magnitude. About 20 different observatories participated in the project, each assigned a narrow strip of Declination to cover. The project was simply too unwieldy, and was completed only in a few of the zones. However, nearly all the observatories got as far as publishing raw x,y measurements of stars from the photographic plates taken for the atlas, totalling some four or five million stars. These printed volumes take up over six meters of shelf space. It turns out the accuracy of these x,y measurements is quite good enough to be still useful a century later. What they provide is a very long time baseline from which to determine the proper motions for stars. The USNO-Washington folks have done this work, and the results are now becoming available. These old observations aren't really of that much use, however, without some new positions to go along with them.
What the Naval Observatory have done is to prepare a new star catalogue combining the old AC x,y positions (reduced to RA/Dec) and new positions measured on plates taken with a 20cm Twin Astrograph in Washington DC. This catalogue obviously doesn't cover the whole sky (yet), but reaches to -18 Dec. In this area are some 750,000 stars, nominally complete to photo-blue mag. 10.5, with a lot of stars included that are up to one magnitude fainter. This is triple the areal density of the ACRS or the PPM, and further, the accuracy of the positions is three times better, approaching 0".1! This is limited now mainly by the fundamental reference frame of positions on the sky. You can download the "Twin Astrograph Catalogue" (TAC) in one-degree strips directly from the USNO at:
http://aries.usno.navy.mil/ad/tac.html
It's three times larger than the PPM, so have plenty of disc space ready! Be sure to read the introductory text at the Web site, since there are some caveats on the quality and completeness of the data for certain zones. This Web page also has a link to the original AC re-reductions, too. There's some interesting historical reading provided about the origins of the AC. The TAC doesn't include all the stars contained in the AC re-reductions, but new positions at similar precision for these fainter stars can be obtained from another up-to-the-minute catalogue: GSC, version 1.2. This is a re-reduction of the GSC using this time the PPM star catalogue as the reference net, and also taking into account very carefully the distortions of the Schmidt plates. The GSC v1.2 is not available yet 'in toto', but can be explored on a star-by-star basis at the Web site for the "Catalogues and Surveys Branch" (CASB) at Space Telescope:
http://www-gsss.stsci.edu/gsc/gsc12/gsc12_form.html
The claim is that the GSC v1.2 is reliable at the 0".3 level, and this appears to be the case for several examples tested by asteroid occultation predictions. Thus the data are five to ten times better than v1.1! The main CASB Web area homepage is at:
...which contains other interesting stuff.
Even the GSC is small change nowadays, thanks to complete deep scans of the original sky survey plates in both hemispheres in two colors. How deep? Magnitude 20. How big? How does 500,000,000 stars sound?! That's something like one-half percent of _all_ the stars in the Milky Way galaxy! Compared to the GSC, this is like a hippopotamus squatting on a pocket Bible. The ten degree square region centered on the Large Magellanic Cloud contains about 15,000,000 stars---as many stars as are in the entire GSC. The sky survey that's readily available now has been produced by the U. S. Naval Observatory's Flagstaff station. The products from the "Proper Motion Machine" (PMM, not PPM!) are described at another USNO Web site:
This page includes links to three on-line search engines (CDS, ESO, Lowell) as well as third-party software for reading the data. There are two main catalogues. "SA1.0" is a selected list of a mere 54 million stars between mag. 16 and 19 uniformly distributed around the sky. It is intended to be used as an astrometric reference net for large telescopes and narrow-field instruments, such as for amateurs doing asteroid and comet astrometry with commercial CCDs. Often there are only a few GSC stars in a field, not really enough to get a good asteroid position. This catalogue will allow even small chips to get enough reference stars to do such measurements. Obviously, this is not a catalogue to use to make star charts, since the stars are chosen by their distribution, not for an inventory of the sky. If you do astrometry, head to the Web page above to read about getting a copy of the CD this catalogue is on, although it is now being included in astrometric reduction software packages.
The catalogue everyone should be excited about is "A1.0". This is a real inventory of the Schmidt sky-survey plates, done from the original plates as far as possible, and reaching even fainter than you can see on the POSS prints, comprising 488,000,000 'detections', with some star/nonstar discrimination. Using the catalogue we have on-line at Lowell, I have extracted positions for mag. 19 variable stars in the thickest parts of the Aquila and Cygnus Milky Way. This comes on 10 CDs (6 gigabytes), and will not be generally available (at least for now) except to professionals, partly because USNO-Flagstaff is not in the business of spending their time cutting CD-ROMs to sell, but mainly because of the complicated legal entanglements involved.
Because a lot of people have their fingers in the pie, it is not clear how these can be used in commercial products. Besides the Navy, there's also the National Geographic (who's paid for both surveys in the north), the European Southern Observatory and the Anglo-Australian Observatory (for the south), SERC/PPARC, Cal Tech, and Space Telescope Science Institute, all of whom have claims to various intellectual property rights and copyrights on the source material. As the project maestro Dave Monet has said, "don't make me wake up the lawyers". However, because the material can be freely searched on-line, I don't see that this is much of a hindrance for ordinary amateur visual/CCD observing.
Catalogues of the near future (Hipparcos/Tycho, Millenium Star Atlas) :
If you talk to anybody making star catalogues now, they'll all tell you that everything is going to be swept aside by Hipparcos. The Hipparcos spacecraft operated in the early 1990s to obtain parallaxes, positions, plus B and V magnitudes for stars. The results had been under tight wraps, but were publicly released in May 1997. The parallax part of the mission (Hipparcos) produced parallaxes good to about 1 milliarcsecond (one thousandth of an arcsecond) and high-precision proper motions for about 100,000 bright stars. Another instrument on the same spacecraft, called Tycho, has produced positions of lesser (but still high) precision plus B and V magnitudes for one million stars---complete to mag. 10.5, and lots of stars to 11.5 (not quite as complete as the TAC).
You can do star-by-star searches using the CDS-Strasbourg "VizieR" look-up facility:
http://vizier.u-strasbg.fr/cgi-bin/VizieR
This page produces a catalogue "order form". In item 1 here, type in the catalogue number, which is I/239. Submit this, then on the succeeding page select either the Hipparcos or Tycho portion of the catalogue, and continue. This brings you to the search page, which allows searches by star name or by position, and selection of what data you would like to see. The entire dataset is also available via ftp from the CDS:
http://cdsweb.u-strasbg.fr/cgi-bin/Cat?I/239
The files come to 427Mb, so the VizieR service is likely to be more useful for most folks.
As a result of using the Hipparcos/Tycho data for a few months, I can offer the following advice. First, note especially that the positions supplied are for epoch 1991.25 but equinox 2000. Because the time baseline of the positions was only a few years, the proper motions derived from the Tycho data are rather poor compared to up-to-date astrometric catalogues mentioned above. The Hipparcos motions seem to be very good, and conform to those shown in the PPM and TAC. Thus I would avoid using the proper motions given by Tycho unless the errors are substantially smaller than the motions themselves. For most purposes it is also better to use positions for both epoch and equinox 2000, such a those in the PPM.
The Hipparcos parallaxes are the best available, but bear in mind that most stars are far enough away that the errors of these observations are a substantial fraction of the parallax itself. For only about 20,000 of the 120,000 Hipparcos stars is the parallax error 10 percent or less of the parallax. In other words, for most stars the parallax distance is merely competitive with other methods of distance determination. For some the data show only that the star is not in the room with you, and sometimes not even that when the errors are larger than the parallax itself. (There are also a fair number of spurious negative parallaxes.) When using the data, be sure to check the value given for the uncertainty.
The VizieR search results also show various data resulting from the photometry done by the instruments. The corrected V magnitudes shown as one of the earlier columns of data seem to be adjusted very closely to the standard V system. Also given in later columns are the raw data, uncorrected for what appears to be a strong color term in the B and V results. The individual magnitudes here seem to be "wrong", but the B-V colors listed seem to be correct when compared to Landolt equatorial standard stars. Again, be sure to look at the errors listed, as these can be quite large for fainter stars in the Tycho catalogue. At mag. 10, the per-observation error of the magnitudes was about 0.4 magnitudes. Only by having made one hundred or more observations was the error of the mean beat down to an acceptable level. The large per-observation errors however, mean that small-amplitude variables among the fainter stars went unnoticed, "lost in the noise".
Many variable-star observers/hunters have despaired that the Hipparcos results have mopped up all the bright variable stars that are not already known. I can confirm, however, that this is simply not so. Yes, there are some 8000 new variables detected, but because the data were taken in a temporally very irregular fashion, not every variable was caught, particularly eclipsing binaries of the Algol type for bright stars, and just about any type fainter than mag. 9.0 or so. Plenty of work still to be done! Most importantly for observers is that Hipparcos is not observing _now_.
More interesting for most amateur observers is that a new large-scale star atlas, the "Millenium Star Atlas", is being produced from the Tycho data by Roger Sinnott and colleagues at Sky Publishing. When it comes out later this year, it will likely be a significant advance on the Uranometria and Herald-Bobroff atlases. (Is it going to be perfect? No.) Advertising for this has already appeared, where further details can be sought. You can find out more generally about the Hipparcos/Tycho mission and its products at the ESA Web site:
http://astro.estec.esa.nl/SA-general/Projects/Hipparcos/hipparcos.html
The high-precision positions from Hipparcos/Tycho mean that all previous ground-based fundamental reference frames are obsolete. Now that the data are available to mere mortals, _everyone_ will re-reduce their position catalogues using the Hipparcos stars as the reference frame. The GSC will be redone again, A1.0 will become A2.0 (or something), the Twin Astrograph Catalogue will get re-reduced; new star catalogues will be started to extend the high-precision to fainter limits. Already the USNO-Washington has planned a digital sky survey to 15th magnitude that will have the same accuracy as Hipparcos. This is scheduled to start in Chile in 1998.
You read/hear about a lot of amazing stuff coming out of astronomy, but it's mostly about specific objects, or an obscure new discovery. Here are some things that will really change the way we do even amateur astronomy. Watch this space!
Brian Skiff ( bas@lowell.edu )
Lowell Observatory
1400 West Mars Hill Road
Flagstaff AZ 86001-4499
USA